1. Field of the Invention
The present invention relates to an EL (electroluminescence) display (an electro-optical device) formed by preparing an EL element on a substrate. More particularly, the invention relates to an EL display using a semiconductor element (an element using a semiconductor thin film). Furthermore, the present invention relates to an electronic device in which the EL display is used in a display portion thereof.
2. Description of the Related Art
In recent years, technology for forming a TFT on a substrate has been largely improved, and an application development of the TFT to an active matrix-type display device has been carried out. In particular, the TFT using a polysilicon film has a higher electric field effect mobility than the TFT using a conventional amorphous silicon film thereby the TFT may be moved at a high speed. Therefore, the pixel control which has been conducted at a driver circuit outside of the substrate may be conducted at the driver circuit which is formed on the same substrate as the pixel.
Such an active matrix-type display device can, by preparing various circuits and elements on the same substrate, obtain various advantages such as a decrease in the manufacturing cost, a decrease in the size of the display device, an increase in the yield ratio, and a decrease in the throughput.
Further, research on the active matrix-type EL display having an EL element as a self-light-emitting device is becoming more and more active. The EL display is referred to as an organic EL display (OELD) or an organic light-emitting diode (OLED).
The EL display is a self-light-emitting type diode unlike a liquid crystal display device. The EL element is constituted in such a manner that an EL layer is sandwiched between a pair of electrodes. However, the EL layer normally has a lamination structure. Typically, the lamination structure of a “positive hole transport layer/a luminous layer/an electron transport layer” proposed by Tang et al. of the Eastman Kodak Company can be cited. This structure has a very high light-emitting efficiency, and this structure is adopted in almost all the EL displays which are currently subjected to research and development.
In addition, the structure may be such that on the pixel electrode, a positive hole injection layer/a positive hole transport layer/a luminous layer/an electron transport layer, or a positive hole injection layer/a positive hole transport layer/a luminous layer/an electron transport layer/an electron injection layer may be laminated in order. Phosphorescent dye or the like may be doped into the luminous layer.
In this specification, all the layers provided between the pair of electrodes are generally referred to as EL layers. Consequently, the positive hole injection layer, the positive hole transport layer, the luminous layer, the electron transport layer, the electron injection layer or the like are all included in the EL layers.
Then, a predetermined voltage is applied to the EL layer having the above structure from the pair of the electrodes, so that a recombination of carriers is generated in the luminous layer and light is emitted. Incidentally, in this specification, the fact that the EL element is emitted is described as the fact that the EL element is driven. Furthermore, in this specification, the anode, the light-emitting element formed of the EL layer and the cathode is referred to as an EL element.
A problem in the practical application of the EL display is the short life of the EL element resulting from the deterioration of the EL layer. As factors which affect the length of the life of the EL layer, the structure of a device which drives the EL display, the characteristic of the organic EL material which constitutes the EL layer, the material of the electrode, and the conditions in the manufacture process or the like can be cited.
Then, in addition to the factors described above, what is recently noted as a factor which affects the length of life of the EL layer is a method for driving the EL display.
Conventionally, in order to emit light from the EL element, a method for applying a direct current to two electrodes, an anode and a cathode sandwiching the EL element has been generally used. The conventional digital style time division gray scale display will be explained by referring to FIG. 16. Here, the case of providing a 2n gray scale full color display with an n-bit-digital drive system will be explained.
FIG. 15 shows a structure of the EL display pixel portion. A gate signal lines (G1 through Gn) to which a gate signal is inputted are connected to the gate electrode of the switching TFT 1501 incorporated in each of the pixels. Furthermore, one of the source region or the drain region of the switching TFT 1501 incorporated in each of the pixels is connected to source signal lines (which are referred to also as data signal lines) (S1 through Sn), while the other is connected to a gate electrode of the EL driving TFT 1504 incorporated in each of the pixels and a capacitor 1508 incorporated in each of the pixels, respectively.
One of the source region and the drain region of the EL driving TFT 1504 incorporated in each of the pixels is connected to the power source supply lines (V1 through Vn) while the other is connected to the EL element 1506. The potential of the power source supply lines (V1 through Vn) is referred to as the potential of the power source. Note that, the power source supply lines (V1 through Vn) is connected to a capacitor 1508 incorporated in each of the pixels. Note that, the digital data signal refers to a digital video signal.
The EL element 1506 comprises an anode and a cathode and an EL layer provided between the anode and the cathode. In the case where the anode is connected to the source region and the drain region of the EL driving TFT 1504, namely, in the case where the anode is the pixel electrode, the cathode which is the opposite electrode is held at a constant potential. On the contrary, in the case where the cathode is connected to the source region or the drain region of the EL driving TFT 1504, that is, in the case the cathode is the pixel electrode, the anode, which is an opposite electrode is held at a constant potential.
Furthermore, in this specification, the potential of the opposite electrode is referred to as a stationary potential. Note that, the power source for giving the stationary potential to the opposite electrode is referred to as a stationary power source. It is desirable that the potential of the anode is higher than the potential applied to the cathode. Therefore, the stationary potential changes in accordance with the fact that the opposite electrode is the anode or the cathode. For example, in the case where the opposite electrode is the anode, it is desirable that the stationary potential is set to be higher than the power source potential. On the contrary, in the case where the opposite electrode is the cathode, it is desirable that the stationary potential is set to be higher than the power source potential.
A difference in the potential between the stationary potential of the opposite electrode and the power source potential of the pixel electrode is the EL driving voltage, and this EL driving voltage is applied to the EL layer.
FIG. 16 shows a timing chart in a digital style driving with a direct current in the conventional EL display. In the beginning, one frame period is divided into n sub-frame periods (SF1 through SFn). Note that, a period in which all the pixels in the pixel portion display one image is referred to as one frame period (F). In a normal EL display, a frame period is provided in which the oscillation frequency is 60 Hz or more, that is, 60 or more frame period per one second is provided, so that 60 or more images are displayed in one second. When the number of images displayed in one second becomes 60 or less, flickering of images such as a flicker or the like becomes visually conspicuous. Note that, the period into which one frame period is further divided into a plurality of periods is referred to as a sub-frame period. With an increase in the number of gray scale levels, the division number of one frame period also increases, and the driver circuit must be driven at a high frequency.
One sub-frame period is divided into an address period (Ta) and a sustain period (Ts). The address period is time required for inputting data to all the pixels during one sub-frame period, while the sustain period (which is also referred to as a lighting period) is the period in which the EL element is lit.
The length of the address periods incorporated in each of the n sub-frame periods (SF1 through SFn) are the same. The sustain periods (Ts) incorporated in sub-frame periods SF1 through SFn respectively are set to Ts1 through Tsn, respectively.
The length of the sustain periods is set to be Ts1: Ts2: Ts3: . . . : Ts(n−1): Tsn=20: 2−1: 2−2: . . . : 2−(n−2): 2−(n−1). However, the order in which the sustain periods SF1 through SFn are allowed to appear may be any. With the combination of this sustain periods, a desired gray scale display can be provided out of 2n gray scale levels.
In the beginning, in the address period, the power source supply lines (V1 through Vn) can be held at the same height with the stationary potential. In this specification, the power source potential in the digital driving address period is referred to as an off power source potential. Note that, the height of the off power source potential should be on the same level with the stationary potential within the scope in which the EL element 1506 does not emit light. Note that, the EL driving voltage at this time is referred to as an off EL driving voltage. It is desired that the EL driving voltage at the OFF time is 0 V, but the voltage may be on the order of not allowing the EL element 1506 to emit light.
Then, the gate signal is inputted to the gate signal line G1, so that the switching TFTs 1501 having the gate electrode connected to the gate signal line G1, are all turned on.
Then, in the state in which the switching TFTs 1501 having the gate electrode connected to the gate signal line G1 is on, the digital data signal is inputted to the source signal lines (S1 through Sn) in order. The digital data signal have information of “0” or “1”, and the digital data signal of “0” and “1” refers to a signal which has either Hi voltage or Lo voltage. Then, the digital data signal inputted to the source signal lines (S1 through Sn) is inputted to the gate electrode of the EL driving TFTs 1504 via the switching TFTs 1501 in the ON state. Furthermore, the digital data signal is inputted to the capacitor 1508 to be held.
Next, the gate signal is inputted to the gate signal line G2, and all the switching TFTs 1501 having the gate electrode connected to the gate signal line G2 are turned on. Then, in the state in which the switching TFT 1501 having the gate electrode connected to the gate signal line G2 is turned on, the digital data signal is inputted to the source signal lines (S1 through Sn) in order. The digital signal inputted to the source signal lines (S1 through Sn) is inputted to the gate electrode of the EL driving TFTs 1504 via the switching TFT 1501. Furthermore, the digital data signal is also inputted to the capacitor 1508 to be held.
The above operation is repeated, and the digital data signal is inputted to all the pixels. The period in which the digital data signal is inputted to all the pixels is an address period.
When the address period is completed, simultaneously the sustain period begins. When the sustain period begins, the potential of the power source supply lines (V1 through Vn) changes from the OFF power source potential to the ON power source potential. In this specification, in the case of the digital driving, the power source potential in the sustain period is referred to as the on power source potential. The difference in potential of the on power source potential and the stationary potential should be such that the EL element emits light. Note that this potential difference is referred to as on EL driving potential. The off power source potential and the on power source potential are generally referred to as the power source potential. Furthermore, the on EL driving voltage and off EL driving voltage are generally referred to as EL driving voltage.
In the sustain period, the switching TFTs 1501 are turned off. Then, the digital data signal held in the capacitor 1508 is inputted to the gate electrode of the EL driving TFTs 1504.
In the case where the digital data has information of “0”, the EL driving TFT 1504 is turned off, so that the pixel electrode of the EL element 1506 is held at the off power potential. As a consequence, the EL element 1506 incorporated in the pixel to which digital data signal having information of “0” is applied, does not emit light.
On the other hand, in the case where the digital data has information of “1”, the EL driving TFT 1504 is turned on, so that the pixel electrode of the EL element 1506 becomes the on power source potential. As a consequence, the EL element 1506 incorporated in the pixel to which digital data signal having information of “1” is applied, emits light.
The period in which all the switching TFTs 1501 are turned off is the sustain period.
The EL element emits light in any of the periods Ts1 through Tsn. In the period of Tsn, a predetermined EL element is allowed to emit light (a predetermined pixel is lit).
Next, the address period appears again. After the digital data signal is inputted to all the pixels, the sustain period appears. At this time, any of the sustain periods Ts1 through Tsn(n−1) appears. Here, Ts(n−1) appears, and a predetermined pixel is allowed to be lit in the Ts(n−1) period.
Thereafter, the similar operation in the remaining n−2 sub-frames is repeated, and the sustain periods Ts(n−2), Ts(n−3) . . . Ts1 appear one after another, and the predetermined pixel is allowed to be lit in the sub-frame.
When n sub-frame periods appear, one frame period is completed. At this time, the gray scale level of the pixel can be determined by summing up the sustain period in which the pixel is lit in one frame period, namely the length of the sustain period immediately after the address period in which the digital data signal having the information of “1” is applied to the pixel.
For example, in case of n=8, when the luminance of the pixel which emits light in all the sustain periods is set to 100%, the luminance of 75% can be represented in the case where the pixel emits light in Ts1 and Ts2. In the case where Ts3, Ts5 and Ts8 are selected, the luminance of 16% can be represented.
In this manner, the conventional EL display is driven with direct current, and the EL driving voltage applied to the EL layer always has the same polarity.
However, as has been introduced in “TSUTSUI. T, Jpn. J. Appl. Phys. Part 2 VOL. 37, NO. 11B, p. L1406-L1408, 1998”, it has been found that the deterioration of the current-voltage characteristic of the EL element has been improved by applying the EL driving voltage having the opposite polarity to the EL element for each period.
However, no concrete proposal has been made with respect to a method for driving an EL display which utilizes the fact that the deterioration of the current-voltage characteristic of the EL element is improved by applying an EL driving voltage having the opposite polarity to the EL element for each period, and with respect to the EL display using the driving method.
Then, in order to prolong the life of the EL element, a proposal on the method for driving the EL display for providing a display (hereinafter referred to as an alternate current driving in this specification) by applying the EL driving voltage having the opposite polarity to the EL element for each of the definite period, and manufacture of the EL display using the driving method has been expected. In particular, manufacture of an active matrix-type EL display for providing a display with the alternate current drive has been expected.